356 MA]!^TJAL OF THE MOLLUSCA. 



arrangement of the muscles. The articulated group possess an 

 anal aperture ; the unarticulated none. (Hancock.)* 



The valves are both opened and closed by muscles ; those 

 which open the shell [cardinales] originate on each side the 

 centre of the ventral valve, and converge towards the hinge- 

 margin of the free valve, behind the dental sockets, where 

 there is usually a prominent cardinal process. The teeth form 

 the fulcrum on which the dorsal valve turns. The adductor 

 muscles are four in number, and quite distinct in Crania and 

 Discina ; in Lingula the posterior pair are combined, and in 

 Terebratula the four muscles are separate at their dorsal 

 terminations, but united at their insertion in the centre of the 

 larger valve. The pedicle is fixed by a pair of muscles (each 

 doubly- attached) to the dorsal hinge-plate, and by another 

 pair to the ventral valve, outside the cardinal muscles. f 



In the Terebratulidse and the other Brachiopods having 

 articulated valves the muscular system consists of 3 pairs of 

 muscles which act directly on the valves, and of 3 pairs which 

 connect the shell, and adjust it with respect to the peduncle. 

 In the unarticulated Brachiopods, such as Lingula, the 

 muscles are more complicated than in the former group ; three 

 pairs of protractor muscles keep the valves together, and thus 

 compensate for the absence of the hinge and condyles, which 

 help to form this function in the articulated group ; they are so 

 arranged as to co-operate in preventing any displacement of 

 the valves in any direction. Hence the term sliding-muscles 

 which they have received is inappropriate, since they prevent 

 any sliding action. In the Lamellibranchs the sliding of the 

 valves is admirably guarded against by means of hinges with 

 teeth and sockets ; in Brachiopods the same end is apparently 

 obtained by means of muscles. It has, therefore, been pro- 

 posed to substitute the ievxn^adjustor iov protractor, and retractor 

 for sliding as applied to these muscles. The following table 

 shows the names in general use, and those proposed by Mr. 

 Hancock : — 



Names in use. Names proposed. Homologous muscles in 



Unarticulated brachiopods. articulated bracliiopoda. 



Ant. retractors. Ant. occlusors. Ant. occlnsors. 



Ant. adductors. Post. „ Post, occlusors. 



■ Post. „ Divaricators. Accessory divaricators. 



* Philosophical Transactions. 1858. 



t The muscular sj'-stem of Terebratula presents a considerable amoimt of rescai- 

 .blnnce to that of Modiola (Fig. 214) ; the anterior and posterior pedal muscles may be 

 compared to the dorsal and vcnti-al pedicle muscles. 



